Introduction: You can store mileage rates in Admin > Setup > Expense > Mileage Rate Sets.
Description: Each Mileage Rate Set can contain multiple rates (based on effective date for each rate entry). Usually each rate represents one year.
It is very important to understand that each year as mileage rates change the only thing you need to do is to create a new row in the schedule of rates. You do NOT need to and should not create a new Mileage Rate set every year (as you would also need to remap the expense type to that mileage rate set.)
Examples of needs for actual multiple Mileage Rate Sets (vs multiple rates within a Set) include: US vs. Canadian travel, personal vehicle vs company vehicle, employee vs. sub-contractor rates). It is common, however, to have only one Mileage Rate Set per system.